FIG. 9 illustrates a MOS drive circuit as an example of a device for driving a load controllable by current of this type. In the figure, n pieces of LEDs 1.sub.1 to 1.sub.n, which are elements controllable by current, are connected in parallel to form a light emitting element array or a display array. In this example, an LED element is used as an element controllable by current. To each of the elements controllable by current, output stage MOS transistors 2.sub.1 to 2.sub.n are connected in series and driving current for the LED is supplied to each element through the output stage MOS transistors. Between the terminals of the source and the gate of the output stage transistors, switching transistors 3.sub.1 to 3.sub.n are connected. A power source voltage Vcc is divided by resistors R1 and R2 for dividing voltage at a point B for obtaining divided voltage, and between the point B for obtaining divided voltage and the gate A of the output stage transistors, the switching elements 4.sub.1 to 4.sub.n are provided. A controller 5 controls turning on and turning off the switching transistors 3.sub.1 to 3.sub.n and the switching elements 4.sub.1 to 4.sub.n on the basis of the data input signal DIN (Data signal IN) from a microcomputer (not shown).
Operations of driving an LED of the above-mentioned conventional driving circuit will be described. For example, to cause the LED 1.sub.1 to turn on, the switching element 4.sub.1 is turned on by the output signal from the controller 5 on the basis of the data input signal DIN. By this operation, the divided voltage, 3 volts for example, of the point B for obtaining divided voltage is supplied to the gate A of the output stage transistor 2.sub.1. Further, since the switching transistor 3.sub.1 is made to be open at this time by the output signal of the controller 5 and the power source voltage Vcc cannot be supplied to the gate of the output stage transistor 2.sub.1, only the divided voltage is supplied to the gate A, and the output stage transistor is made to be on. Thus a driving current is supplied to the LED element 1.sub.1 which is connected in series to the output stage transistor, and the LED element is turned on. To turn off the element, the switching transistor 3.sub.1 is made to be off by the output signal of the controller 5 (the switching element 4.sub.1 is turned off at this time). By this operation, the voltage applied to the gate A of the output stage transistor 2.sub.1 is boosted to the power source voltage Vcc to exceed a conduction threshold voltage, 4 volts for example, of the output stage transistor making the output stage transistor open, and the driving current is stopped to flow to the LED element 1.sub.1, and the LED element is turned off. In the manner as described above, by the output signal from the controller 5 on the basis of the data input signal DIN, on-off control of the output stage transistors 2.sub.1 to 2.sub.n are performed, and turning on and turning off of each LED element is controlled.
Operations for turning on the above-mentioned output stage transistors 2.sub.1 to 2.sub.n will be described referring to FIG. 10. When the voltage applied to the gate A drops from the power source voltage Vcc (5 volts) to a level further below from the above-mentioned conduction threshold voltage (4 volts) and to the divided voltage (3 volts) of the point B for obtaining divided voltage, the output stage transistor is made to be in an ON state from an OFF state.
In order to change the output stage transistor from the OFF state to the ON state in a manner described above, the time necessary for the applied voltage of the gate so as to change from 5 volts to 3 volts is calculated approximately as about 2.6 .tau. from the time .tau. which is decided by the product .tau. (.tau.=RC) of the output impedance of the voltage source, that is, a resistance R (combined resistance of R1 and R2) of the point B for obtaining divided voltage and the total internal capacity C (gate capacity) of the output stage transistor.
The gate capacity of each output stage transistor itself is approximately 3 pF for example, but normally an array configuration is formed in which approximately 100 output stage transistors are connected in parallel as the output stage transistor for the driving circuit, so that the total gate capacity of the output stage transistors 2.sub.1 to 2.sub.n is approximately 300 pF. The resistance R of the point B for obtaining divided voltage is several kilo-ohms, and assuming that the resistance R of the point B of divided voltage is 2 k.OMEGA. for example, the time .tau. of the above is: .tau.=2.times.10.sup.3 .times.300.times.10.sup.-12 =600 nsec. Therefore, the actual delay time of operating to change the output stage transistor from OFF state to ON state is approximated to about 2.6 .tau. (=1560 nsec.).
As above described, since the output impedance of the point B for obtaining divided voltage due to dividing resistance or capacity of the output stage transistor are relatively large and since the combined capacity of the output transistors becomes large by providing the parallel array of the output stage transistors, there are problems that the speed for the output stage transistor's turning on is delayed, and that the improvement of the speed for driving the LED elements is prevented by the combined capacity. In addition, a large MOS transistor is necessary for the transistor used in the output stage in consideration of the current capability and countermeasures for electrostatic breakdown, etc. In order to flow the driving current of at least 1 mA, for example, if a transistor, the design size W/L of which (W: gate width, L: gate length) of 640/6 is used, the capacity in this case is increased to 4 pF, which is an obstacle to the improvement of the driving speed of the LED element. In particular, as described in the case of the above example, when the time .tau. is close to approximately 600 nsec., such time is so long that the conventional driving apparatus is not suitable for practical use in driving a light emitting element array or a display array of a video game or the like, for example.